1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the scanning optical apparatus. More particularly, the present invention relates to a scanning optical apparatus that is suitably used for an apparatus, such as a laser beam printer or a digital copying machine having an electrophotographic process, in which an optically modulated light flux emitted from a light source means is reflected and deflected by a polygon mirror functioning as an optical deflection means and then optically scans a surface to be scanned through an scanning optical system, thereby recording image information. In particular, the present invention relates to a scanning optical apparatus with which there is always obtained a favorable image where the sensitivity of scanning line bending to rotational decentration of a single lens constituting a scanning optical system is reduced. The present invention also relates to an image forming apparatus using the scanning optical apparatus.
Also, the present invention relates to a color image forming apparatus that uses a plurality of scanning optical apparatuses and is constructed from a plurality of image bearing members corresponding to respective colors.
2. Related Background Art
In a conventional scanning optical apparatus applied to a laser beam printer (LBP) or the like, a light flux optically modulated in accordance with an image signal and emitted from a light source means is periodically deflected by a light deflector composed of a rotary polygon mirror (polygon mirror) or the like. The deflected light flux is converged to form a spot on a surface of a photosensitive recording medium (photosensitive drum) by an imaging scanning optical system having an fxcex8 characteristic and optically scans the surface, thereby performing image recording.
FIG. 20 is a schematic diagram showing the main part of a conventional scanning optical apparatus.
In this drawing, a diverging light flux emitted from a light source means 171 is converted into a nearly parallel light flux by a collimator lens 172, an aperture stop 173 limits the light flux, and the limited light flux strikes a cylindrical lens 174 having a predetermined refractive power only in the sub scanning direction. The nearly parallel light flux striking the cylindrical lens 174 is emitted as it is in a main scanning cross-section. Also, in a sub scanning cross-section, the light flux is converged and imaged as a nearly linear image on a deflecting surface (a reflecting surface) 175a of a light deflector (a deflection means) 175 composed of a polygon mirror.
Then, the light flux deflected by the deflecting surface 175a of the light deflector (the deflection means) 175 is guided by an imaging scanning optical system 76 having an fxcex8 characteristic onto a photosensitive drum surface 178 that is a surface to be scanned. By having the light deflector (the deflection means) 175 rotate in the direction of arrow A, the photosensitive drum surface 178 is optically scanned in the direction of arrow B, thereby performing the recording of image information.
In order to perform the recording of image information with high precision in a scanning optical apparatus like this, it is required that the following requirements are met. For instance, the curvature of field is favorably corrected across the entire of the surface to be scanned. Also, there exists a distortion characteristic (fxcex8 characteristic) having a uniform speed property between an angle of view xcex8 and an image height Y. Further, the spot diameter on an image surface remains uniform irrespective of differences in image height. There have conventionally been proposed various kinds of scanning optical apparatuses or imaging scanning optical systems that satisfy optical characteristics like these.
On the other hand, as the sizes and prices of apparatuses, such as laser beam printers and digital copying machines, are reduced, the same demand is made to scanning optical apparatuses.
As a construction satisfying the demand like this, JP 04-50908 A and JP 09-33850 A, for instance, propose a scanning optical apparatus whose imaging scanning optical system is constructed from a single fxcex8 lens.
In JP 04-50908 A, an aberration characteristic is relatively favorably corrected by using an aspheric surface of a high order in the main scanning direction of an fxcex8 lens. However, the magnification between a deflection means and a surface to be scanned in the sub scanning direction does not remain constant, so that there is a tendency for a spot diameter in the sub scanning direction to change in accordance with the differences in image height.
On the other hand, in JP 09-33850 A, there is proposed a method described below. In a scanning optical apparatus, on at least two surfaces of lens surfaces of an fxcex8 lens, the curvature in the sub scanning direction continuously changes along the main scanning direction within an effective region of the fxcex8 lens as well as independently of the curvature in the main scanning direction. With this method, the position of the principal plane in the sub scanning direction is controlled by the bending of two surfaces. In this manner, the sub scanning magnification is kept constant irrespective of the differences in image height, so that there is obtained a constant spot diameter.
With the proposed method described above, in order to obtain a constant sub scanning magnification, at least two surfaces are bent and the position of the principal plane is controlled so that the magnification is kept constant. Consequently, it becomes possible to set shapes in the main scanning direction and the sub scanning direction completely independently of each other. However, because of various demands such as a demand to reduce the thickness of a lens, the lens shape in the main scanning direction has a relatively large amount of an aspheric surface in many cases.
In a lens like this whose amount of an aspheric surface in the main scanning direction is large, optical performance is significantly degraded due to errors caused during the arrangement of each lens surface and the lens. Among the degradations of optical performance, as distinct from the aberration of the height of a scanning line, the inclination of the scanning line, and the like, the bending of the scanning line in the sub scanning direction in particular causes a significant problem because it is impossible to correct this bending by the adjustment of a mirror or the like arranged in an apparatus main body. Consequently, in order to suppress the scanning line bending to a minute amount, it is required to precisely arrange each lens surface and the lens in accordance with design values or to adjust the positions of each lens surface and the lens so as to coincide with design positions by providing the lens with an adjustment mechanism.
Further, in the case of a color image forming apparatus in which scanning optical apparatuses are arranged using four photosensitive members (photosensitive drums), latent images are formed by laser light, and images on an original in respective colors of Y (yellow), M (magenta), C (cyan), and Bk (black) are formed on the surfaces of their corresponding photosensitive members, images in four colors of Y, M, C, and Bk formed on the surfaces of respective photosensitive members are superimposed on each other on a transfer member such as paper. Consequently, if the scanning lines of the scanning optical apparatuses corresponding to respective photosensitive members are bent, there occur differences in shape between scanning lines for the four colors, which causes a problem that color drift occurs in an image on the transfer member and therefore image performance is significantly degraded.
The present invention has been made to solve these problems and an object of the present invention is to provide a scanning optical apparatus that is capable of suppressing scanning line bending caused by rotational decenterity of a single lens to a minute amount and an image forming apparatus using the scanning optical apparatus. To do so, an fxcex8 lens is constructed using a single lens and the shape of the fxcex8 lens is appropriately set so that there are favorably corrected the field curvature characteristic that is a characteristic of an optical system, an fxcex8 characteristic for performing uniform speed scanning, and wave front aberration.
According to a first aspect of the present invention, there is provided a scanning optical apparatus comprising: light source means; deflection means for deflecting a light flux emitted from the light source means; and a scanning optical system that images the deflected light flux as a spot on a surface to be scanned, in which the scanning optical system is a single lens, and a surface shape of the single lens is set so that a direction of scanning line bending in a sub scanning direction occurring when an incident surface of the single lens is rotationally decenter about an axis parallel to a main scanning direction is opposite to a direction of scanning line bending in the sub scanning direction occurring when an exit surface of the single lens is rotationally decenter about the axis parallel to the main scanning direction.
According to a second aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, the scanning line bending in the sub scanning direction occurring when the incident surface of the single lens is rotationally decenter about the axis parallel to the main scanning direction cancels out the scanning line bending in the sub scanning direction occurring when the exit surface of the single lens is rotationally decenter about the axis parallel to the main scanning direction.
According to a third aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, when power in the sub scanning direction of entirety of the scanning optical system is referred to as "PHgr"s and power in the sub scanning direction of the exit surface of the single lens is referred to as "PHgr"s2, the power in the sub scanning direction of the exit surface of the single lens satisfies a condition of 0.9 "PHgr"s2/"PHgr"s 1.1.
According to a fourth aspect of the present invention, in the first aspect of the invention, there is provided a scanning optical apparatus, in which power in the sub scanning direction of an exit surface of the single lens satisfies a condition of 0.95 "PHgr"s2/"PHgr"s 1.05.
According to a fifth aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, when an air converted distance from the deflection means to the exit surface of the single lens on an optical axis is referred to as Lao, a distance from the exit surface of the single lens to the surface to be scanned is referred to as Lbo, an off-axis air converted distance from the deflection means to the exit surface of the single lens is referred to as Laxcex8, and a distance from the exit surface of the single lens to the surface to be scanned is referred to as Lbxcex8, a shape of the exit surface of the single lens in the main scanning direction satisfies the following condition:       0.9    xc3x97                  L        bo                    L        ao              ≤            L              b        ⁢                  xe2x80x83                ⁢        θ                    L              a        ⁢                  xe2x80x83                ⁢        θ              ≤      1.1    xc3x97                  L        bo                    L        ao            
According to a sixth aspect of the present invention, in the optical scanning apparatus according to the fourth aspect of the invention, a shape of the exit surface of the single lens in the main scanning direction satisfies the following condition.
According to a seventh aspect of the present invention, in the scanning optical apparatus according to the first aspect of the invention, a light flux forming a linear image that is long in the main scanning direction is incident on the deflection means.
According to an eighth aspect of the present invention, in the scanning optical apparatus according to the first aspect of the invention, the single lens is formed by a molding process.
According to a ninth aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, at least one of power of the exit surface in the sub scanning direction and power of the incident surface in the sub scanning direction varies without any correlation with a shape in the main scanning direction.
According to a tenth aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, a radius of curvature of the exit surface in the sub scanning direction varies from on the axis toward off the axis.
According to an eleventh aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, each of the incident surface and the exit surface is an anamorphotic surface.
According to a twelfth aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, a shape of the exit surface of the single lens is circular arc.
According to a thirteenth aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, a shape of the exit surface of the single lens is non-circular arc.
According to a fourteenth aspect of the present invention, in the optical scanning apparatus according to the first aspect of the invention, a difference between imaging magnifications in the sub scanning direction of the scanning optical system within an image effective area is 10% or less.
According to a fifteenth aspect of the present invention, in the scanning optical apparatus according to the twelfth aspect of the invention, a shape of the exit surface of the single lens in the main scanning direction is a nearly circular arc shape having a center of curvature on the deflection means side.
According to a sixteenth aspect of the present invention, in the scanning optical apparatus according to the first aspect of the invention, the single lens satisfies the following equations:             "LeftBracketingBar"              Δ        ⁢                  xe2x80x83                ⁢                  Z          l                    "RightBracketingBar"        =                  "LeftBracketingBar"                              Δ            ⁢                          xe2x80x83                        ⁢                          Z                              1                ⁢                d                                              +                      Δ            ⁢                          xe2x80x83                        ⁢                          Z                              1                ⁢                L                                              +                      Δ            ⁢                          xe2x80x83                        ⁢                          Z                              1                ⁢                φ                                              +                      Δ            ⁢                          xe2x80x83                        ⁢                          Z              2                                      "RightBracketingBar"            ≤      0.040                  Δ      ⁢              xe2x80x83            ⁢              Z        2              =          Δ      ⁢              xe2x80x83            ⁢              X        2            xc3x97                        L                      2            ⁢            o                                    fs                      2            ⁢            o                              xc3x97      γ                  Δ      ⁢              xe2x80x83            ⁢              Z                  1          ⁢          d                      =                            N          -          1                N            xc3x97              1                              cos            2                    ⁢          φ                    xc3x97              (                  1          -                                    L                              2                ⁢                o                                                    fs                              2                ⁢                o                                                    )            xc3x97      γ      xc3x97              (                              d            θ                    -                      d            o                          )                        Δ      ⁢              xe2x80x83            ⁢              Z                  1          ⁢          L                      =                  (                  N          -          1                )            xc3x97              1                              cos            2                    ⁢          φ                    xc3x97      γ      xc3x97              (                              L                          2              ⁢              θ                                -                      L                          2              ⁢              o                                      )                        Δ      ⁢              xe2x80x83            ⁢              Z                  1          ⁢          φ                      =                  (                  N          -          1                )            xc3x97              L                  2          ⁢          o                    xc3x97      γ      xc3x97              tan        2            ⁢      φ      
where xcex94X2 is a deviation of an exit surface 6b of the lens,
Rxo is a distance from the exit surface of the lens to a center of sagittal curvature on an optical axis along a direction of the optical axis,
Rxxcex8 is a distance from the exit surface of the lens to the center of sagittal curvature at an angle of view xcex8 along the direction of the optical axis,
L2o is a distance from the exit surface of the lens to the surface to be scanned on the optical axis,
L2xcex8 is a distance from the exit surface of the lens to the surface to be scanned at an angle of view 74,
fs2o is a focal length of the exit surface of the lens in the sub scanning direction on the optical axis,
fs2o is a focal length of the exit surface of the lens in the sub scanning direction at the angle of view xcex8,
N is a refractive index of the lens,
do is a distance between the incident surface and the exit surface of the lens on the optical axis,
dxcex8 is a distance between an incident surface 6a and the exit surface of a lens 6 at the angle of view xcex8,
fs2o is a focal length of the exit surface of the lens in the sub scanning direction on the optical axis,
fs2xcex8 is a focal length of the exit surface of the lens in the sub scanning direction at the angle of view xcex8,
L2o is a distance from the exit surface of the lens to the surface to be scanned on the optical axis,
L2xcex8 is a distance from the exit surface of the lens to the surface to be scanned at the angle of view xcex8, and
"PHgr" is an inclination of a ray of light after emission from the incident surface in the main scanning direction at the angle of view xcex8.
According to a seventeenth aspect of the present invention, in the scanning optical apparatus according to the first aspect of the invention, the light source means is a multi-beam light source having a plurality of light-emitting points that can be modulated independently of each other.
According to an eighteenth aspect of the present invention, there is provided an image forming apparatus comprising: a scanning optical apparatus according to any one of the first to seventeenth aspects of the invention; a photosensitive member arranged on the surface to be scanned; a developing device that develops an electrostatic latent image formed on the photosensitive member by a light flux scanned by the scanning optical apparatus as a toner image; a transferring device that transfers the developed toner image onto a material to be transferred; and a fixing device that fixes the transferred toner image on the material to be transferred.
According to a nineteenth aspect of the present invention, there is provided an image forming apparatus comprising: a scanning optical apparatus according to any one of the first to seventeenth aspects of the invention; and a printer controller that converts code data inputted from an external device into an image signal and inputs the image signal into the scanning optical apparatus.
According to a twentieth aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of scanning optical apparatuses that are each a scanning optical apparatus according to any one of the first to seventeenth aspects of the invention; and a plurality of image bearing members that are arranged on surfaces to be scanned of respective scanning optical apparatuses and form images in colors differing from each other.